Field of the Invention
The invention relates to a method and a membrane module for process-integrated oxygen generation during biomass gasification, wherein the oxygen is generated at high temperature via mixed conducting ceramic membranes.
Discussion of Background Information
Biomass gasification with air yields a nitrogen-containing synthesis gas having a calorific value generally no higher than 1.8 kWh per cubic meter SCM (standard cubic meter according to DIN 1343). This value cannot be surpassed even when using ideal qualities of raw material, e.g., dry beechwood chips. The gas engines developed for synthesis gases and mixed gases attain only low electrical efficiencies with lean gases of this kind such that power generation is not competitive. In order to increase the calorific value, gases with a higher calorific value, e.g., biomethane or natural gas, can be mixed in, but this appreciably increases fuel costs. Therefore, as an alternative to gasification with air, larger gasification installations are operated with oxygen. Owing to the resulting high gasification temperatures, steam is often mixed in with the oxygen for cooling (WO 2008/068596 A2).
Depending on the type of gasifier employed, the synthesis gas from biomass gasification contains various percentages of tar which must be removed before utilization in the combined heat and power plant (referred to hereinafter throughout as CHP plant). A reliable method for tar removal is to reheat the synthesis gas to approximately 1200° C., which can be achieved relatively easily by adding oxygen to the hot synthesis gas. Further, oxygen can also be used to increase the electrical output of gas engines or fuel cells, e.g., by increasing the oxygen content of the supplied combustion air.
Conventional production of oxygen is preferably carried out through pressure swing adsorption (PSA) or cryogenic air separation (Linde® process). Energy-optimized large-scale plants achieve minimum specific energy consumptions of 0.4 kWhel./m3 SCM O2 (cryogenic) or 0.36 kWhel./m3 SCM O2 (PSA). However, biomass gasification requires only comparatively small amounts of oxygen which are commonly provided through smaller PSA installations. These PSA installations need appreciably more than 1.0 kWhel./m3 SCM O2 and, therefore, as a result of their own electrical power requirement, considerably reduce the economic return to the gasification plant or render it uneconomical. The use of oxygen from tanks or liquid storage tanks entails considerable expenditures for rental and transportation and has therefore also failed to gain popularity up to this point.
An alternative method for the production of oxygen is based on a membrane separation process at high temperatures. Mixed conducting ceramic membranes (MIEC—Mixed Ionic Electronic Conductors) are used for this purpose and enable a highly selective separation of oxygen. The oxygen transport relies on the transporting of oxide ions through the gastight ceramic material and the transporting of electronic charge carriers (electrons or electron holes) taking place simultaneously. Since the 1980s, a great number of ceramic materials have been investigated with respect to oxygen transport and further material characteristics (Sunarso, J., Baumann, S., Serra, J. M., Meulenberg, W. A., Liu, S., Lin, Y. S., Diniz da Costa, J. C.: Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation, J. Membrane Sc. 320 (2008), 13-41).
Oxygen permeation through an MIEC membrane can be described by Wagner's equation and is determined primarily through the ambipolar conductivity of the material at operating temperature, the membrane thickness and through the driving force. The latter is given by the logarithmic ratio of oxygen partial pressure in the feed gas (ph) to oxygen partial pressure in the sweep gas (pi) or in the permeate. Consequently, in a given material with constant membrane thickness and fixed temperature, the oxygen flux through a MIEC membrane is proportional to In(ph/pi). Accordingly, doubling ph on the feed gas side results in the same increase in oxygen flux as halving pi on the permeate side or sweep gas side. Consequently, in order to generate pure oxygen in plants utilizing membrane technology, the air can be compressed or the oxygen can be sucked out by vacuum. Of course, combined processes are also possible (Armstrong, P. A., Bennett, D. L., Foster. E. P., Stein. V. E.: The New Oxygen Supply for the New IGCC Market, Gasification Techn. 2005, San Francisco, 9-12 Oct. 2005). Compression of air is preferred for commercial plants because compressors are generally cheaper and more available than vacuum generators.
If the generated oxygen is needed for chemical reactions, the driving force can be generated most favorably in terms of energy by the sweeping of the MIEC membrane with low-oxygen gases. The oxy-coal AC process (http://www.oxycoal-ac.de/index.php?id=1099&L=0) for a coal-fired power plant, i.e., the combustion of coal in a CO2/O2 mixture, uses the recirculated flue gas as sweep gas at the MIEC membrane because it has oxygen contents of only 1-3 percent by volume. To increase the oxygen flux through the membrane, the air is compressed on the feed gas side and the compression energy is largely recovered downstream of the membrane through an expansion turbine. Minimizing energy losses requires a high efficiency of the compressor and turbine. Further, an external pressure vessel is required in order to realize a favorable load condition of the ceramic membrane components.
Currently available MIEC membrane materials with high oxygen permeation are unstable under CO2 because the alkaline earths contained therein form carbonates with the CO2 and block the membrane surface (Schulz, M., Kriegel, R., Kämpfer, A.: Assessment of CO2 stability and oxygen flux of oxygen permeable membranes, J. Membr. Sc. 378 (2011), pages 10-17). For this reason, processes with no sweep gas have been developed as an alternative to the oxy-coal AC process with CO2 sweep, also known as 4-end process. These alternative processes are referred to as dead-end or 3-end processes. For this purpose, as has already been mentioned, pure oxygen is generated by generating pressure differentials.
In the field of power generation, a number of patents have claimed the use of MIEC membranes for oxy-fuel combustion in coal-fired plants with the aim of CO2 separation (WO 2009/065374 A3, EP 2 026 004 A1). Various method schemes aim primarily to minimize the expected efficiency losses as far as possible. In WO 2009/065374 A3, in contrast to the usually preferred overpressure processes, vacuums are applied on the permeate side. This makes possible a membrane module without high-temperature-resistant external pressure vessels, and less compression energy is consumed because only the oxygen on the permeate side of the membrane need be compressed. It is disadvantageous that the compression energy cannot be recovered in the overall process.
WO 2008/014481 and EP 2 067 937 A2 claim the generation of oxygen via MIEC membrane materials and the use thereof in the gasification power plant. In both patents, the air entering the membrane module is compressed and the compression energy is recovered through expansion turbines.
For biomass gasification, the use of MIEC membrane materials for oxygen generation is only meaningful when the energy consumption can be reduced substantially below that of typical PSA plants. The own energy requirement for MIEC membrane separation results on the one hand from the thermal energy required for maintaining the high temperature of 800-900° C. at the membrane. On the other hand, compression energy for gas compression is needed to generate the driving force for oxygen transport. To the detriment of thermotechnical integration, MIEC membrane material materials with high oxygen flux known heretofore could usually only be used within a limited temperature range. Accordingly, below approximately 830° C., the commonly used material BSCF is prone to a slow phase decomposition resulting in diminished oxygen permeation (Shao, Z., Yang, W., Cong, Y., Dong, H., Tong, J., Xiong, G.: Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen membrane, J. of Membr. Sc. 172 (2000), pages 177-188). Moreover, the creep rate of the material increases with increasing temperature, and mechanical failure of the membrane components can come about due to the pressure differences occurring at the membrane (Pecanac, G., Baumann, S., Malzbender, J.: Mechanical properties and lifetime predictions for Ba0.5Sr0.5Co0.8Fe0.2O3-δ membrane material, J. of Membr. Sc. 385-386 (2011), pages 263-268). Moreover, direct contact with synthesis gas leads to higher corrosion (Lu, H., Tong, J., Cong, Y., Yang, W.: Partial oxidation of methane in Ba0.5Sr0.5Co0.8Fe0.2O3-δ membrane reactor at high pressures, Catalysis Today 104 (2005), pages 154-159) and, in some cases, to fracture of the membrane. A direct heating of the membranes with combustion gases also appears impossible because the CO2 content in the flue gas leads to the membrane being covered with alkaline earth carbonates and to blockage of the oxygen permeation (Arnold, M., Wang, H., Feldhoff, A.: Influence of CO2 on the oxygen permeation performance and the microstructure of perovskite-type (Ba0.5Sr0.5)Co0.8Fe0.2O3-δ membranes, J. of Membr. Sc. 293 (2007), pages 44-52).
Accordingly, the disadvantageous characteristics and limited conditions of use of highly-developed MIEC membrane material materials enumerated above lead to considerable limitations in the technical realization of a process-integrated membrane module for oxygen generation. There remains only the possibility of tempering the gas flows entering the membrane module in a correspondingly exact manner or providing the membrane module with additional electric heating in order to ensure operation in the optimal temperature range and to prevent contact with gases having a corrosive effect. However, this would result in elaborate, highly complex plant controls or in high additional consumption of electrical power.
It is the object of the invention to provide a possibility for energy-efficient oxygen generation in biomass gasification for increasing the efficiency of the overall process.